CHAPTER XXIV
SLIPPERINESS AND ITS TREATMENT
THE problem of " skidding " on wet road surfaces has exercised the
minds of all those interested in the construction of roads and the
design of vehicles. The increasing number of motor vehicles
travelling upon our highways, coupled with the continual wet
weather experienced during recent years, makes the question one of
great urgency.
It is perhaps expedient to examine briefly the principal causes of
skidding before discussing remedies. f
The onus would appear to lie with the road engineer rather than
with the vehicle designer, as the condition of the road surface in wet
weather is primarily responsible for a skid.
The following may be said to be the principal causes of the
trouble:—
1. Smooth, fine road surfaces of bituminous asphalte, having a
low coefficient of friction when wet; caused too, by wear and
by hot weather.
2. Excessive camber or crown.
3. Camber construction of roads at bends also on slow curves,
4. Corrugation or waviness of the surface.
5. Light rainfall which moistens the foreign matter (such as oil)
on the surface, without scouring it.
6. Insufficient weight on the rear axle of the vehicle, or lack of
balance in loading.
7. Back-axle driving and braking, under the action of the differ-
ential gear.
8. A sudden change in the nature of the road surface.
9. Worn tyres, or unequal inflation.
10. Driving in " top-gear " on obviously*slippery and dangerous
road surfaces.
Remedies.
Generally, with bituminous surfaces it is a good plan (a) to have
suitable bituminous compositions which have the maximum per-
missible proportion of large aggregate; (b) to roll plain or pre-coated
chippings into newly laid surfaces or (c) to flush and/or to grit greasy
surfaces.
With regard to concrete paving the finish of the surface is most
important in this connection,
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